The two in Cabot House (4)
I believe the answer is:
both
'the two' is the definition.
(synonyms)
'in cabot house' is the wordplay.
'in' indicates a hidden word.
'BOTH' is hidden in the letters of 'cabot house'.
(Other definitions for both that I've seen before include "The two (things)" , "The one and the other" , "Two considered together" , "Each of two (alternatives)" , "couple" .)